In-Operando Oxygen Evolution Reaction Electrocatalysis in Acidic Media

Abstract

The transition from fossil fuels to renewable sources requires the utilization of green hydrogen. Electrolysis is one of the areas gaining support within both government and industry spheres, due to the possibility of producing hydrogen using renewable energy sources at a high gas purity and current density. Despite significant interest in proton-exchange membrane (PEM) electrolysis, the oxygen evolution reaction (OER) at the anode is considered one of the main bottlenecks due to the sluggish electrode kinetics and the instability of many electrocatalysts in acidic electrolytes. This is because it is a four electron reaction with a high activation barrier, and so new active electrocatalysts are needed, which in turn needs a mechanistic understanding to target new materials. In-operando studies can be used to advance the mechanistic understanding of the reaction and shed light into rate-limiting steps as well as elucidate on potential structural changes of the electrocatalysts during the reaction.Here, we report on a modified electrochemical cell for in-operando X-ray diffraction (XRD) studies of the OER using a calcium iridium pyrochlore (Ca2-xIr2O6·H2O) electrocatalyst spray-coated onto Toray carbon paper. 1 Whilst electrochemical techniques such as chronoamperometry and cyclic voltammetry are utilized in acidic electrolyte (0.1 M H2SO4), changes to the catalyst surface and degradation rates are monitored by XRD. This provides an overall picture of phase and crystallinity changes of the iridium pyrochlore throughout the OER. X-ray absorption near edge spectroscopy (XANES), complementary to XRD, is carried out to further determine the iridium oxidation state changes during the catalytic process. The catalyst is furthermore assessed ex-situ by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) in both, pre- and post-electrochemical experiments, to examine changes to its morphology and surface composition. Appropriate cell modifications accounting for mass transport as well as gas bubble formation at the working electrode surface are developed following commercial electrolysis cell designs. This allows studying both, catalyst activity and stability, under realistic operation conditions that is currently unprecedented in the literature.